Which of these numbers is prime? ${12,\ 17,\ 51,\ 65,\ 77}$
Solution: A prime number has exactly two factors: 1 and itself. The factors of 12 are 1, 2, 3, 4, 6, and 12. The factors of 17 are 1 and 17. The factors of 51 are 1, 3, 17, and 51. The factors of 65 are 1, 5, 13, and 65. The factors of 77 are 1, 7, 11, and 77. Thus, 17 is a prime number.